1. Field of the Invention
The invention generally relates to ultrasonic surgical systems and, more particularly, to a method for detecting transverse mode vibrations in an ultrasonic hand piece/blade.
2. Description of the Related Art
It is known that electric scalpels and lasers can be used as a surgical instrument to perform the dual function of simultaneously effecting the incision and hemostatis of soft tissue by cauterizing tissues and blood vessels. However, such instruments employ very high temperatures to achieve coagulation, causing vaporization and fumes as well as splattering. Additionally, the use of such instruments often results in relatively wide zones of thermal tissue damage.
Cutting and cauterizing of tissue by means of surgical blades vibrated at high speeds by ultrasonic drive mechanisms is also well known. One of the problems associated with such ultrasonic cutting instruments is uncontrolled or undamped vibrations and the heat, as well as material fatigue resulting therefrom. In an operating room environment attempts have been made to control this heating problem by the inclusion of cooling systems with heat exchangers to cool the blade. In one known system, for example, the ultrasonic cutting and tissue fragmentation system requires a cooling system augmented with a water circulating jacket and means for irrigation and aspiration of the cutting site. Another known system requires the delivery of cryogenic fluids to the cutting blade.
It is known to limit the current delivered to the transducer as a means for limiting the heat generated therein. However, this could result in insufficient power to the blade at a time when it is needed for the most effective treatment of the patient. U.S. Pat. No. 5,026,387 to Thomas, which is assigned to the assignee of the present application and is incorporated herein by reference, discloses a system for controlling the heat in an ultrasonic surgical cutting and hemostasis system without the use of a coolant, by controlling the drive energy supplied to the blade. In the system according to this patent an ultrasonic generator is provided which produces an electrical signal of a particular voltage, current and frequency, e.g. 55,500 cycles per second. The generator is connected by a cable to a hand piece which contains piezoceramic elements forming an ultrasonic transducer. In response to a switch on the hand piece or a foot switch connected to the generator by another cable, the generator signal is applied to the transducer, which causes a longitudinal vibration of its elements. A structure connects the transducer to a surgical blade, which is thus vibrated at ultrasonic frequencies when the generator signal is applied to the transducer. The structure is designed to resonate at the selected frequency, thus amplifying the motion initiated by the transducer.
The signal provided to the transducer is controlled so as to provide power on demand to the transducer in response to the continuous or periodic sensing of the loading condition (tissue contact or withdrawal) of the blade. As a result, the device goes from a low power, idle state to a selectable high power, cutting state automatically depending on whether the scalpel is or is not in contact with tissue. A third, high power coagulation mode is manually selectable with automatic return to an idle power level when the blade is not in contact with tissue. Since the ultrasonic power is not continuously supplied to the blade, it generates less ambient heat, but imparts sufficient energy to the tissue for incisions and cauterization when necessary.
The control system in the Thomas patent is of the analog type. A phase lock loop (that includes a voltage controlled oscillator, a frequency divider, a power switch, a matching network and a phase detector), stabilizes the frequency applied to the hand piece. A microprocessor controls the amount of power by sampling the frequency, current and voltage applied to the hand piece, because these parameters change with load on the blade.
The power versus load curve in a generator in a typical ultrasonic surgical system, such as that described in the Thomas patent, has two segments. The first segment has a positive slope of increasing power as the load increases, which indicates constant current delivery. The second segment has a negative slope of decreasing power as the load increases, which indicates a constant or saturated output voltage. The regulated current for the first segment is fixed by the design of the electronic components and the second segment voltage is limited by the maximum output voltage of the design. This arrangement is inflexible since the power versus load characteristics of the output of such a system can not be optimized to various types of hand piece transducers and ultrasonic blades. The performance of traditional analog ultrasonic power systems for surgical instruments is affected by the component tolerances and their variability in the generator electronics due to changes in operating temperature. In particular, temperature changes can cause wide variations in key system parameters such as frequency lock range, drive signal level, and other system performance measures.
In order to operate an ultrasonic surgical system in an efficient manner, during startup the frequency of the signal supplied to the hand piece transducer is swept over a range to locate the resonance frequency. Once it is found, the generator phase lock loop locks on to the resonance frequency, continues to monitor the transducer current to voltage phase angle, and maintains the transducer resonating by driving it at the resonance frequency. A key function of such systems is to maintain the transducer resonating across load and temperature changes that vary the resonance frequency. However, these traditional ultrasonic drive systems have little to no flexibility with regards to adaptive frequency control. Such flexibility is key to the system""s ability to discriminate undesired resonances. In particular, these systems can only search for resonance in one direction, i.e., with increasing or decreasing frequencies and their search pattern is fixed. The system cannot: (i) hop over other resonance modes or make any heuristic decisions, such as what resonance to skip or lock onto, and (ii) ensure delivery of power only when appropriate frequency lock is achieved.
The prior art ultrasonic generator systems also have little flexibility with regard to amplitude control, which would allow the system to employ adaptive control algorithms and decision making. For example, these fixed systems lack the ability to make heuristic decisions with regards to the output drive, e.g., current or frequency, based on the load on the blade and/or the current to voltage phase angle. It also limits the system""s ability to set optimal transducer drive signal levels for consistent efficient performance, which would increase the useful life of the transducer and ensure safe operating conditions for the blade. Further, the lack of control over amplitude and frequency control reduces the system""s ability to perform diagnostic tests on the transducer/blade system and to support troubleshooting in general.
Some limited diagnostic tests performed in the past involve sending a signal to the transducer to cause the blade to move and the system to be brought into resonance or some other vibration mode. The response of the blade is then determined by measuring the electrical signal supplied to the transducer when the system is in one of these modes. The ultrasonic system described in U.S. Application Ser. No. 09/693,621, filed on Oct. 20, 2000, which is incorporated herein by reference, possesses the ability to sweep the output drive frequency, monitor the frequency response of the ultrasonic transducer and blade, extract parameters from this response, and use these parameters for system diagnostics. This frequency sweep and response measurement mode is achieved via a digital code such that the output drive frequency can be stepped with high resolution, accuracy, and repeatability not existent in prior art ultrasonic systems.
Another problem associated with the prior art ultrasonic systems is unwanted vibrations in the hand piece/blade. Ultrasonic blades also vibrate along an axis which is perpendicular to the longitudinal axis of vibration of the hand piece/blade. Such vibrations are called transverse mode vibrations. If the longitudinal vibration is considered to be in the Z direction in an X, Y, Z coordinate system, vibrations along a Y-axis of the blade are called transverse xe2x80x9cflap modexe2x80x9d vibrations and vibrations along an X-axis of the blade are called transverse xe2x80x9chook modexe2x80x9d vibrations. Blades typically have a sheath surrounding their blade part.
Transverse mode vibrations generate heat, which leads to high blade and/or blade sheath temperatures. This can damage tissue surrounding an indented narrow cut or coagulation zone, thus adversely affecting patient healing and recovery time. In addition, transverse mode vibrations can cause blade tip failures. The vibrations may also be indicative of defects in the hand piece, such as damaged transducer disks. While excess transverse mode vibrations are sometimes annoyingly audible, often a user will ignore them for as long as possible. It is therefore advantageous to detect transverse mode vibrations to prevent undesired effects, such as tissue damage which can occur from an over heated blade.
The invention is a method for detecting transverse mode vibrations in an ultrasonic transducer/blade. With this method, the power delivered to the hand piece/blade is monitored at multiple power levels to determine whether it changes as expected when the power levels applied to the hand piece/blade are changed. (Power levels are associated with specific currents at which the generator drives the hand piece/blade, regardless of load changes on the blade.)
Transverse mode vibrations are excited by (non-linear) interactions of longitudinal vibrations with the hand piece/blade. These vibrations bend the blade, thereby causing the generation of heat, which drains energy from the desired longitudinal vibrations. This energy drain manifests itself as an increase in hand piece/blade impedance, thereby necessitating an increase in the power delivered to the hand piece/blade to maintain the required current through the transducer of the hand piece.
The non-linear mechanical coupling of energy from longitudinal to transverse vibrations is appreciable only above certain energy/displacement thresholds. Therefore, if the impedance seen at a low xe2x80x9creferencexe2x80x9d power level is less than the impedance of the same hand piece/blade at a higher power level, then transverse vibrations are more than likely present. In embodiments, a power measurement at a low power level under test is used to calculate an expected power at a high power level.
A reference power consumption measurement at a low power level setting is performed. This measurement is used to establish pass/fail power consumption levels for a high power level setting. The reference power level measurement is essential, because the transducer/blade impedance seen by the generator depends on the blade used.
In accordance with the invention, while the blade is being held in midair, the power delivered to the hand piece/blade is measured at a low power level setting, where the drive current is low and does not trigger transverse vibrations. Using the value obtained at the low drive current level setting, the expected power at a second high power is calculated and used to set a pass/fail threshold for a second measurement at the high power level setting. Next, the actual power delivered to the hand piece/blade is measured at the high power level setting, and a determination is made whether the hand piece/blade exhibits transverse mode vibrations based on whether the actual measured power at the high power level setting exceeded the established pass/fail threshold level. If this is the case, operation of the generator is inhibited, a xe2x80x9cTransverse Mode Vibrations Present in Hand Piece/Bladexe2x80x9d error code is stored in the generator, and a xe2x80x9cBad Hand Piecexe2x80x9d message is displayed on the LCD of the console.
In accordance with an embodiment, while the blade is being held in midair, the drive current level is swept from a minimum drive current to a maximum drive current. During the current sweep, the transducer voltage and current drive signals are monitored and stored in non-volatile memory located in the generator. Using the stored voltage and current data, the power delivered to the transducer is calculated, and the Power-Delivered vs. Drive Current and Hand Piece/Blade Impedance vs. Drive Current response curves are generated. Using the generated response curves, an extrapolation is performed to determine whether the Hand Piece/Blade exhibits transverse mode vibrations. If this is the case, operation of the generator is inhibited, a xe2x80x9cTransverse Mode Vibrations Present in Hand Piece/Bladexe2x80x9d error code is stored in the generator, and a xe2x80x9cBad Hand Piecexe2x80x9d message is displayed on the LCD of the console.
In an alternative embodiment, a Multiple Level Drive Power vs. Power Delivered relationship and/or a Multiple Level Drive Power vs. Impedance relationship is used to detect or predict potential transverse mode problems, along with an xe2x80x9cover-drivexe2x80x9d of the hand piece at one or several power drive levels beyond the normal range of power levels used. These xe2x80x9cover-drivexe2x80x9d power levels are particularly effective at rapidly identifying problematic or potentially problematic transverse mode conditions.
In another embodiment, the power delivered to the hand piece is measured at multiple frequencies while a high power drive signal is applied to the hand piece. Alternatively, an xe2x80x9cover-drivexe2x80x9d is used. Here, three frequencies, i.e., a first frequency, a second frequency and a third frequency, are measured in close proximity to each other. The first frequency is the primary resonance frequency, otherwise referred to as the main or intended longitudinal resonance frequency of the hand piece/blade. The second frequency is slightly below the first frequency. The third frequency is slightly above the first frequency. The expected impedance or power increases somewhat at both the second and third frequencies. If the impedance or power is substantially higher or higher than expected, this condition indicates the presence of a transverse resonance, which may create undesired heat and/or reduce ultrasonic energy delivered into tissue. In this case, an alert or alarm is generated by the generator console. If necessary, a handicap limited functionality or complete disabling of the hand piece drive is performed.
Instead of monitoring impedance or power delivered to the hand piece, other variables can be monitored for comparison, such as the phase, the current, the voltage, a power factor, or the like. Alternately, rather than comparing the primary frequency measurements to both a slightly higher and a slightly lower frequency measurement, the comparison is performed at only the xe2x80x9csecond frequencyxe2x80x9d or at only the xe2x80x9cthird frequency.xe2x80x9d As a result, the monitoring process is accelerated in cases where monitoring additional frequencies is not necessary or needed.
In another embodiment of the invention, while the blade is being held in midair or on tissue, the power delivered to the hand piece/blade is measured at first and second frequencies. Using the values obtained at the first and second frequencies, the expected power at a third frequency, a fourth frequency and a fifth frequency are calculated and used to set a pass/fail threshold level for an actual measured third power through an actual measured fifth power, respectively. Next, the actual power delivered to the hand piece/blade is measured at the third, fourth, and fifth frequencies. A determination is made whether the hand piece/blade exhibits transverse mode vibration based on whether any of the actual measured powers exceed the established pass/fail threshold levels. If this is the case, operation of the generator is inhibited, and an alarm/alert message and/or audible alarm/alert is generated. Rather than monitoring the power delivered to the hand piece, in alternative embodiments other variables are monitored for comparison purposes, such as the phase, the impedance, the current, the voltage, the power factor, the phase margin, or the like.
In a further embodiment of the invention, the occurrence of whether a transverse frequency is located near the intended drive resonance is determined, and difficulties associated with the detection of transverse modes stimulated by the primary/main resonance drive frequency at high power are resolved.
The method provides an indication of whether a hand piece, which failed the power level tests, will exhibit transverse vibrational modes if it is used. In addition, the method eliminates the need to know the specific type of blade being used with the hand piece during diagnostic testing.